Tag Archives: Cisco

The previous post in this series looked at the exchange of routes between BGP peers, and this post takes this a step further by looking at two BGP attributes that can be modified to influence how a router decides which routes to place in it’s routing table – WEIGHT and LOCAL PREFERENCE Here’s our example […]

The first post in this BGP mini-series looked at setting up BGP peering between routers. Now we’ll take things a step further and actually exchange some routing information between those peers. Here’s the network: The basic BGP config on each router looks like this: R1#sh run | s router router bgp 1 no synchronization bgp […]

This is the first post in a mini-series on BGP basics, and looks at setting up internal and external BGP neighbours using loopback interfaces. The advantage loopbacks have over physical interfaces is that they are always up and reachable. iBGP configuration: The routers must be able to reach each others loopback IP’s and we don’t […]

One of the golden rules of OSPF is that all areas must be connected to the backbone area 0, however sometimes this is not physically possible, and this is where Virtual Links come in. A virtual link is created through another area that is connected to area 0. To explain this, here is our network: […]

When multiple OSPF routers are connected to a multi-access medium such as Ethernet, a Designated Router (DR) and a Backup Designated Router (BDR) are elected. DR’s reduce network traffic as only they maintain the complete ospf database and then send updates to the other routers on the shared network segment. The other routers become ‘slaves’ […]

We know that two OSPF routers will become neighbours if they can agree on certain parameters (see my previous post for details) and this is great, but suppose the bad guys were able to connect their own router to the same data link and then cause OSPF routing mayhem – how do we stop this??? […]

In this post we’re going to take a look at the process of two Cisco routers forming an OSPF neighbour adjacency, with an emphasis on what’s going on at a packet level. We’ll look at the packets sent and received on a multi-access network type, in this case Ethernet. Here’s our network: Our OSPF config […]

There are many reasons why you would want to implement route filtering. A good example would be to prevent routes to transit networks being advertised to a branch router, because no host in the branch would ever need to send packets to a host in the transit network. Filtering in EIGRP is configured using the […]

Stub Routers in EIGRP have two purposes: to prevent a router from advertising any routes it has learnt via EIGRP to neighbouring routers to limit the scope of query messages when a route goes “active” Here’s our network (all links 100Mbps, unless stated): Going “active” means that a Successor route to a network has gone […]

In certain situations an engineer may need to manipulate EIGRP metrics, and one method is to use Offset Lists to increase both the AD (Advertised Distance) and FD (Feasible Distance) of a route by a certain value – the offset. Offset List configurations define the following: route(s) that we want to amend the metric for […]